1. Field of the Invention
The present invention relates to a differential pressure detecting apparatus which is made small in size, light in weight and low in price, and in which a welded portion of a contact portion with an enclosed liquid is designed not to contact with outside air, so that the welded portion is prevented from being corroded even if the outside air is corrosive.
2. Description of the Related Art
A conventional example will be described with reference to FIG. 3 showing its sectional view. A cover 45 is fixed by not-shown mounting screws onto each of left and right sides of a body 41 through an O-ring 46, and an upper case 47 is welded onto the upper surface of the body 41. The body 41 has left and right end surfaces parallel to each other. A closed-end hole 41a countersunk with two stages is formed in the left end surface while a hole 4lb is formed in the right end surface coaxially with the hole 41a. An electrostatic capacity type differential pressure detection portion 1 is inserted into a hole portion of the body 41, and leads 2 from the detection portion 1 are led out upward to the inside of the upper case 47. The upper case 47 includes an amplifier, various kinds of arithmetic operation circuits, an indicator, and so on. In addition, a sub-body 43 is inserted into the countersunk portion of the body 41. A seal diaphragm 44 is provided on the left end surface of this sub-body 43, and a hole 43a is formed along the axis of the sub-body 43. A liquid for transmitting pressure is enclosed in the inside space of the body 41, and each hole for filling the liquid is sealed with a ball 23 and a thrust screw 24. A not-shown hole for introducing pressure is formed in each of the right and left covers 45, so that one and the other pressure relating to differential pressure to be measured are introduced therefrom. These respective pressures are transmitted to the differential pressure detection portion 1 through the seal diaphragm 42 and 44 and the enclosed liquid so as to be converted into a change of electrostatic capacity corresponding to the differential pressure. This change of electrostatic capacity is converted into an electric current, by a well known technique, through the amplifier, the arithmetic operation circuits and so on provided in the upper case 47, indicated on the spot, and transmitted to a distant place at the same time.
Another conventional example will be described with reference to FIG. 4 showing its sectional view. In the drawing, this conventional example is roughly constituted by a detection portion 50 and a protection portion 60 which are made to communicate with each other through pressure introducing tubes 51 respectively. The detection portion 50 converts the differential pressure to be measured into an electric signal by way of electrostatic capacity, and outputs the thus obtained electric signal. The protection portion 60, which will be described in detail later, protects the detection portion 50 from the introduced pressures. The structure of this detection portion 50 is well known, and therefore the description thereof is omitted. The protection portion 60 is constituted mainly by left and right bodies 61, a protective diaphragm 65, left and right seal diaphragms 64, O-rings 66 and pressure introducing covers 67. The respective bodies 61 are disposed on the left and right of the protective diaphragm 65, and connected with each other on their outer circumference or circumferential edge portions.
A hollow portion 63 and holes 62 are formed in each of the bodies 61. Description will be made more specifically upon the right body 61 representatively. The hollow portion 63 is formed in the left side surface of the body 61 into a conical shape coaxial with the body 61. One of the holes 62 penetrates the body 61 along the axis of the body 61, while the other hole 62 opens near the center of the hollow portion 63 on the one hand, and, on the other hand, penetrate the pressure introducing tube 51 so as to communicate with a not-shown pressure introducing space of the detection portion 50. The right side surface of the body 61 is formed so as to have a wavy section, and the seal diaphragm 64 having almost the same shape as this wavy section is fixed to the circumferential edge of the right side surface of the body 61 so as to have a gap between the seal diaphragm 64 and the right side surface of the body 61. The cover 67 is attached to the circumferential edge of the right side surface of the body 61 through the O-ring 66. Substantially the same is applied also to the left body 61. Silicon oil (enclosed liquid) as a pressure transmitting liquid is enclosed The inside space of each of the bodies 61.
This conventional example has an effect as follows. If respective introduced pressures (including static pressure) on both sides of a differential pressure type flowmeter such as an orifice meter are received on the respective seal diaphragms 64, the respective introduced pressures are transmitted to their associated pressure introducing spaces of the detection portion 50 through the spaces adjacent to the seal diaphragms 64, the holes 62 and the hollow portions 63 respectively. Each of the seal diaphragms 64 has an extremely small spring constant (that is, it is very soft), while a not-shown detecting diaphragm of the detection portion 50 has an extremely large spring constant (that is, it is very hard). The spring constant of the protective diaphragm 65 takes an intermediate value of the above two spring constants. In the detection portion 50, a differential pressure based on the respective introduced pressures is converted into an electric signal in a well-known electrostatic capacity technique, and the electric signal is outputted. This is the case in which a normal pressure introducing operation is carried out.
However, assuming that only the right seal diaphragm 64 receives one of the introduced pressures by a wrong operation, there is a fear that the detection portion 50 receives a large biased pressure so that the detection portion will be broken if the protective portion 60 is not provided. Even in the case where only one of the introduced pressures is received by one of the seal diaphragms with a wrong operation in the introduction of the introduced pressures on the both sides of the orifice meter, the protection portion 60 protects the detection portion 50 by such an operation as follows. Assume that only the right seal diaphragm 64 receives pressure. Then, on one hand, this pressure is transmitted from the right hole 62 and hollow portion 63 to the left hollow 63 and hole 62 through the enclosed liquid and through the protective diaphragm 65 to thereby expand the left seal diaphragm 64. On the other hand, this pressure is transmitted, through the right hole 62, to the right pressure introducing space of the detection portion 50. However, this transmitted pressure is limited to a certain value or less since the right seal diaphragm 64 contacts with the right wavy surface of the body 61 opposite toe the diaphragm 64. Thus the function to protect the detection portion 50 operates so that there is no fear that the detection portion 50 is broken.
In the former of the above conventional examples, in which the covers 45 for introducing pressure are disposed on both sides of the body 41, the size is therefore increased not only lengthwise but also widthwise. Accordingly, there is a defect that it is inevitable to increase the size, the weight and the cost.
In the latter of the above conventional examples, which is constituted by a protection portion 60 having a protection function from biased pressure and a detection portion 50 installed in the upper thereof, there is a defect that it is inevitable to increase the size, the weight and the cost in the same manner as in the previous conventional example. In addition thereto, since a welded portion of a contact portion with an enclosed liquid, specifically, a welded portion between an outer circumferential portion of a protective diaphragm 65 and an outer circumferential portion of an end surface of each of left and right bodies 61 contacts with outside air, there is a defect that the welded portion is corroded if the outside air is corrosive.